mdatagen: add metric validation test coverage

[Syedowais312]

This PR adds test coverage for metric validation logic in mdatagen, including checks for invalid or incomplete metric definitions and semantic convention URLs.

Follow-up to the stability-focused PR(#14408) to keep test concerns separated.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.84%. Comparing base (d471f6d) to head (e8ac564).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14467      +/-   ##
==========================================
- Coverage   91.85%   91.84%   -0.02%     
==========================================
  Files         677      677              
  Lines       42680    42686       +6     
==========================================
+ Hits        39205    39206       +1     
- Misses       2422     2425       +3     
- Partials     1053     1055       +2     

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[codspeed-hq]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 61 untouched benchmarks
⏩ 20 skipped benchmarks¹

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_{Comparing Syedowais312:test/mdatagen-metric-validation-v2 (e8ac564) with main (3ac3dd7)}

Footnotes

1. 20 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩